The goal of this proposal is to use cryo-electron microscopy to make three-dimensional reconstructions of COPII coated vesicles. COPII proteins are responsible for forming the vesicles that transport proteins and lipids from the endoplasmic reticulum (ER) to the Golgi apparatus. COPII proteins consist of Sar1, Sec23/24, and Sec13/31. These form a lattice on the ER membrane that invaginates the membrane into a spherical vesicle. Biochemical experiments have suggested ways that the proteins interact and have lead to a number of models for the formation of the COPII lattice. The structure of the COPII lattice will be studied by cryoelectron microscopy. Crystal structures of Sar1 and Sec23/24 have been solved. Fitting these structures into the reconstruction of the COPII coat will help determine the inter-protein contacts in the COPII lattice and how the lattice is formed. Moreover, these studies will contribute to the understanding of eukaryotic protein transport machinery and may contribute to the understanding and treatment of diseases associated with disrupted transport.